In many industries, including oil, paper and pulp, textiles, and food processing, various processes produce contaminated water as a by-product. This is especially true in crude oil production and refining because substantially all crude oil is produced from subterranean formations which contain water. This produced water is by the far the largest volume product or waste stream associated with oil and gas production and it presents challenges and costs to operators. Because the water cannot be disposed of by simply dumping it into the environment, numerous methods and systems have been devised to reduce the contaminant content of the water to a level that permits its discharge into the environment without any deleterious consequences.
The basic method of separating a mixture of oil and water is by use of gravity. For this purpose, separators are frequently employed at the point where the crude oil first reaches the earth's surface. These separators range from rather unsophisticated holding vessels—which simply provide an enclosed container wherein the oil and water mixture can rest with reduced turbulence, thereby allowing the oil to float to an upper part of the vessel and water to settle to a lower part of the vessel—to more sophisticated vessels that apply desalting and dehydration methods. Regardless of the type of vessel used, it is common for oil-coated solids (“mud”) to accumulate in the bottom of the vessel and for a mixture of oil and water (“emulsion” or “rag”) to form at the oil and water interface.
Removal of the solids and the emulsion from the vessel is problematic. The customary practice is to allow the solids and emulsion to accumulate until their presence begins to interfere with the overall performance of the vessel. At this point in time, the vessel is shut down so that the solids and emulsion can be extracted and then conveyed to another location for cleaning and processing. Because this batch extraction method disrupts both the oil and water separation process and the production process, a need exists for a method of removing the solids and emulsion on a continuous basis while also processing those contaminants at the source. This processing should occur with equipment that comprises a relatively small footprint.
The processing of these contaminants is also problematic. For example, effluent water often contains organic species that have a high partition coefficient and benzene. Efficient removal of these organic species causes problems for refiners in their waste water treatment operations and difficulty in meeting discharge permit requirements. Current benzene extraction unit designs require high volumes of steam pressure and are prone to fouling from heavy hydrocarbon entrainment in the effluent water. Additionally, the effluent water may contain heavy hydrocarbon interface emulsion that can settle in a refinery process sewer and cause plugging and flow restriction problems. Cleaning processes that remove those restrictions can cause leaks and loss of containment integrity. Hydrocarbon and solids entering the process sewer immediately takes on a hazardous waste designation. Therefore, a need exists for a reliable and efficient method to remove organic species, benzene, and heavy hydrocarbon emulsion from the effluent water so that the treated water discharged to the process sewer meets environmental requirements.